A power conversion system receives an input direct current (DC) or alternating current (AC) power, and converts it to a DC or AC output power, typically exhibiting a different voltage than the input power. Control of the output power may be responsive to the output voltage or to the output current.
A boost converter, also known as a step-up converter, is a power converter with an output voltage greater than its input voltage. It is a class of switching-mode power supply containing at least a first electronically controlled switch, e.g. a transistor, at least a first energy storage element, e.g. an electric coil, and an additional element such as a diode or a second electronically controlled switch. Typically, the electronically controlled switches and diode are arranged between the electric coil and the output, with current being alternately drawn to charge the electric coil responsive to the first electronically controlled switch being closed, and passed to a load responsive to the first electronically controlled switch being open. The current goes through the diode or the second electronically controlled switch when it is passed to the load.
A buck converter, also known as a step-down converter, is a power converter with an output voltage less than its input voltage. It is a class of switching-mode power supply containing at least a third electronically controlled switch, e.g. a transistor, at least a second energy storage element, e.g. an electric coil, and an additional element such as a diode or a fourth electronically controlled switch. Typically, the electronically controlled switches and diode are arranged between the input power source and the electric coil, with current being alternately drawn to charge the electric coil through a load responsive to the third electronically controlled switch being closed, and continued to the load discharging the electric coil responsive to the third electronically controlled switch being open. The diode or the fourth electronically controlled switch is in series with the electric coil when the electric coil is discharging to the load.
A flyback converter is a converter with an output voltage which can be greater than or less than the input voltage. It is a class of switching mode power supply containing: at least one electronically controlled switch; an energy storage element comprising at least one electric coil, specifically a transformer, thereby the voltage ratios are multiplied with an additional advantage of isolation; and at least one additional element, such as a diode and/or additional electronically controlled switches. Typically, the primary electric coil of the transformer is connected between the electronically controlled switch and the input voltage and the secondary electric coil of the transformer is connected between the additional element and the output.
The above listing of power converters is meant to be illustrative of a number of topologies, however this is not meant to be limiting in any way.
Growing demand for high density equipments continuously requires power supplies with higher switching frequencies. The higher the switching frequency, the smaller the power supply for a predetermined rating, and the faster the dynamic response of the power supply. Losses in the various switching topologies are at least partially due to switching losses, i.e. when one of the electronically controlled switches changes state from the off state, i.e. open, to the on state, i.e. closed, and from the on state to the off state. The finite duration of the switching transient of the electronically controlled switch, as well as the non-zero voltage thereacross and the current flowing therethrough, results in switching losses in the switch. As the switching frequency increases, the relevant portion of losses associated with the switching increases.
Zero voltage switching (ZVS), is defined as changing the state of the switch from the off state to the on state when the voltage across the switch is at or near zero. ZVS significantly reduces the amount of switching losses during the closing of the switch.
U.S. Pat. No. 4,672,303 issued Jun. 9, 1987 to Newton, the entire contents of which is incorporated herein by reference, is addressed to a DC/DC converter having reduced switching losses at high frequencies, by providing for zero voltage switching of the buck converter in-line switch. Unfortunately, the arrangement requires that the operating frequency of the DC/DC converter increase with reduced load, otherwise the peak to peak current in the electric coil will be independent of the load, and a large current may be circulated between the input and output even with a small load. U.S. Pat. No. 7,154,250 issued Dec. 26, 2006 to Vinciarelli, the entire contents of which is incorporated herein by reference, is addressed to a Buck-Boost DC-DC Switching Power Conversion apparatus having a clamp phase. Unfortunately, the method of Vinciarelli is not applicable to a broad range of converters since it requires the clamp phase to be supplied by a particular arrangement and timing of the switching devices used to charge and discharge the electric coil.
What is desired, and not provided by the prior art, is a method of achieving zero voltage switching which allows for a fixed operating frequency over a wide range of load and input voltage conditions.